Allelic and Nonallelic Genes Controlling Host Specificity in a Bacteriophage.

ACTERIA infected with any two independently arising r (plaque type) B mutants of the bacterial virus T2H have been found, as a rule, to yield wild-type and double-mutant individuals among the viral progeny ( HERSHEY and ROTMAN 1948). This result has been interpreted in terms of recombination among nonallelic genetic-factors, and it is permissible to speak of the test in which bacteria are infected with two different viral mutants as a genetic cross. Preliminary crosses ( HERSHEY and ROTMAN 1948, and unpublished) between pairs of independently arising host-range mutants of the same virus, however, have failed to yield recombinants, even when the two mutants were clearly different. LURIA (personal communication) has had the same experience with host-range mutants of the related phage T2L. If the results with the two kinds of mutant are to be given a consistent interpretation, it has to be assumed that different host-range mutations, unlike the r mutations, tend to occur at a single genetic locus. This assumption is not implausible, inasmuch as the host-range character can be readily combined with the r character in the appropriate genetic crosses (HERSHEY and ROTMAN 1949). Nevertheless, we felt that the genetic interpretation could be greatly strengthened by establishing one authentic example of multiple allelism in the virus. This we have done, by three independent means. The demonstration is all the more convincing because one mutant was found that proved to be carrying a mutation in a second locus controlling host specificity.